We have learnt about the basic principles of steam generation and its advantages as a heating medium in the context of dairy plants. Now we will go through the different types of steam and how they are different from each other.
i. The Saturated Steam
We know that to generate steam at a given pressure, we have to heat the water in a closed vessel to the saturation temperature of steam generation corresponding to that pressure. Steam thus generated is known as ‘the saturated steam’. If the steam contain water droplets in it, this will make the steam wet. The steam which remains in contact with water is usually wet. This type of steam is called as wet saturated steam. If we separate the water droplets from the steam this will make it dry. This type of steam is known as dry saturated steam. A dry steam thus contains no water droplet in it and a wet steam is a mixture of dry steam and water droplets. In milk plants we usually find the wet steam.
ii. Dryness Fraction of Steam
A wet steam means all of the water in it is not in the vapour form. If in 1 kg. of wet steam there are 0.1 kg of water droplets , the rest 0.9 kg will be the dry steam.The ratio of the weight of dry steam in a given wet steam is known as the ‘dryness fraction’ of the stream. In the above case the dryness fraction of steam would be 0.9. If whole of the steam is dry and there is no water droplet in it, then the dryness fraction of such a steam would be 1.0. In dry steam whole of the water in it will be in the vapour state. The dryness fraction of steam expresses the quality of steam. The more the dryness fraction of the steam, more dry it will be, as more of the water in it will be present in the vapour state. Wet steam could be made dry by passing it through steam separators, in which water droplets are removed from the steam and the steam dries up.
iii. Superheated Steam
Many a times, more heat is provided to the steam, after separating the saturated steam from water, to increase its temperature further. Such a steam is known as superheated steam. Pressure of superheated steam is, however, kept same as that of saturated steam. This is done in a separate device known as Steam Superheater.The super heated steam has higher heat content as compared to saturated steam.
i. The Saturated Steam
We know that to generate steam at a given pressure, we have to heat the water in a closed vessel to the saturation temperature of steam generation corresponding to that pressure. Steam thus generated is known as ‘the saturated steam’. If the steam contain water droplets in it, this will make the steam wet. The steam which remains in contact with water is usually wet. This type of steam is called as wet saturated steam. If we separate the water droplets from the steam this will make it dry. This type of steam is known as dry saturated steam. A dry steam thus contains no water droplet in it and a wet steam is a mixture of dry steam and water droplets. In milk plants we usually find the wet steam.
ii. Dryness Fraction of Steam
A wet steam means all of the water in it is not in the vapour form. If in 1 kg. of wet steam there are 0.1 kg of water droplets , the rest 0.9 kg will be the dry steam.The ratio of the weight of dry steam in a given wet steam is known as the ‘dryness fraction’ of the stream. In the above case the dryness fraction of steam would be 0.9. If whole of the steam is dry and there is no water droplet in it, then the dryness fraction of such a steam would be 1.0. In dry steam whole of the water in it will be in the vapour state. The dryness fraction of steam expresses the quality of steam. The more the dryness fraction of the steam, more dry it will be, as more of the water in it will be present in the vapour state. Wet steam could be made dry by passing it through steam separators, in which water droplets are removed from the steam and the steam dries up.
iii. Superheated Steam
Many a times, more heat is provided to the steam, after separating the saturated steam from water, to increase its temperature further. Such a steam is known as superheated steam. Pressure of superheated steam is, however, kept same as that of saturated steam. This is done in a separate device known as Steam Superheater.The super heated steam has higher heat content as compared to saturated steam.
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